1. Field of the Invention
The present invention is concerned with catalysts useful for the treatment of gases to reduce contaminants contained therein, such as catalysts of the type generally referred to as "three-way conversion" or "TWC" catalysts. TWC catalysts are polyfunctional in that they have the capability of substantially simultaneously catalyzing both oxidation and reduction reactions, such as the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides in a gaseous stream. Such catalysts find utility in a number of fields, including the treatment of the exhaust gases from internal combustion engines, such as automobile and other gasoline-fueled engines.
2. Background and Related Art
In order to meet governmental emissions standards for internal combustion engine exhausts, so-called catalytic converters containing a suitable catalyst such as a TWC catalyst, are emplaced in the exhaust gas line of internal combustion engines to promote the oxidation of unburned hydrocarbons ("HC") and carbon monoxide ("CO") and the reduction of nitrogen oxides ("NO.sub.x ") in the exhaust gas. For this purpose, TWC catalysts comprising a minor amount of one or more platinum group metals distended upon a high surface area, refractory metal oxide support are well known in the art. The platinum group metal may comprise platinum or palladium, preferably including one or more of rhodium, ruthenium and iridium, especially rhodium. The refractory metal oxide support may comprise a high surface area alumina coating (often referred to as "activated" or "gamma" alumina) carried on a carrier such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure, as well known in the art. The carrier may also comprise refractory particles such as spheres or short, extruded segments of a refractory material such as alumina.
The catalytically active materials dispersed on the activated alumina may contain, in addition to the platinum group metals, one or more base metal oxides, such as oxides of nickel, cobalt, manganese, iron, rhenium, etc., as shown, for example, in C. D. Keith et al U.S. Pat. No. 4,552,732. The activated alumina typically exhibits a BET surface area in excess of 60 square meters per gram ("m.sup.2 /g"), often up to about 200 m.sup.2 /g or more. Such activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa and theta alumina phases.
The refractory metal oxide supports are subject to thermal degradation from extended exposure to the high temperatures of exhaust gas resulting in a loss of exposed catalyst surface area and a corresponding decrease in catalytic activity. It is a known expedient in the art to stabilize refractory metal oxide supports against such thermal degradation by the use of materials such as zirconia, titania, alkaline, earth metal oxides such as baria, calcia or strontia or, most usually, rare earth metal oxides, for example, ceria, lanthana and mixtures of two or more rare earth metal oxides. For example, see C. D. Keith et al U.S. Pat. No. 4,171,288.
TWC catalysts are currently formulated with complex washcoat composition containing stabilized Al.sub.2 O.sub.3, an oxygen storage component, primarily ceria, and precious metal catalytic components. Such catalysts are designed to be effective over a specific operating range of both lean of, and rich of, stoichiometric conditions. (The term "oxygen storage component" is used to designate a material which is believed to be capable of being oxidizing during oxygen-rich (lean) cycles of the gas being treated, and releasing oxygen during oxygen-poor (rich) cycles.) Such TWC catalyst compositions enable optimization of the conversion of harmful emissions (HC, CO and NO.sub.x) to innocuous substances. Of the three precious metals, platinum, palladium and rhodium, conventionally used in TWC catalysts, rhodium is the most effective for reducing NO.sub.x to harmless nitrogen. Unfortunately, rhodium is also the most expensive of these costly materials and, consequently, effective rhodium utilization in automotive exhaust catalysts, such as TWC catalysts, has been extensively studied.
One of the problems faced by present-day catalysts is the high operating temperatures engendered by smaller automotive engines and high speed highway driving. Not only alumina support materials as noted above, but oxygen storage components are susceptible to thermal degradation at such high temperatures. Thermal degradation adversely affects the stability of the catalyst and effectiveness of the precious metals used therein. In addition, attempts to improve fuel economy by using air to fuel ("A/F") ratios higher than stoichiometric, and/or fuel shut-off features, generate a lean (oxygen-rich) exhaust. High exhaust gas temperatures and lean gas conditions accelerate the deterioration of platinum and rhodium catalysts, inasmuch as platinum is more readily sintered, and rhodium more strongly interacts with support materials such as alumina, at such conditions.
The art has devoted a great deal of effort in attempts to improve the efficiency of platinum and rhodium-based TWC compositions. Thus, U.S. Pat. No. 4,675,308 discloses a method of effective utilization of rhodium by placing it on alumina which is segregated from ceria-containing particles since ceria enhances the interaction between rhodium and alumina, which renders the rhodium less active.
U.S. Pat. No. 4,806,519 separates the rhodium component in a layered structure in which rhodium is supported on alumina in a second coat which is segregated from the ceria-containing material in a first coat. However, in both cases the rhodium is still primarily in contact with alumina support particles so that any thermal degradation occurring to the alumina will inevitably affect the catalytic efficiency of the rhodium.
The use of layered coatings in catalyst compositions is also shown in two Japanese patent publications. Japanese Patent Application 88-326823/46 (J63240-947A) of Nissan Motor KK (10.02.87-JP-027383) discloses a catalyst comprising a support having two different alumina coatings separately loaded thereon. One alumina coating contains ceria-alumina and ceria on which platinum, palladium of rhodium is dispersed, and is stated to be effective for CO and HC removal. The other alumina coating, which is stated to be effective for NO.sub.x removal, is made from lanthana-alumina and zirconia oxide partially stabilized with Pr and on which palladium or rhodium is dispersed. The catalyst is stated to have TWC activity.
Nissan Motor Company Ltd. Japanese patent publication JP63 77,544 (88 77,544), 7 April 1988, discloses a catalyst comprising a first washcoat containing activated alumina bearing rare earth oxides, and a second washcoat disposed over the first washcoat and containing activated aluminum bearing rare earth oxides, mainly ceria and zirconia. Palladium is kept away from poisonous substances near the washcoat surfaces and forms LA-O-Pd in the first washcoat and Rh-O-Zr in the second washcoat.
Co-pending and commonly assigned U.S. patent application Ser. No. 07/234,226 presents a method to improve thermal stability of a TWC catalyst containing platinum and rhodium by incorporating a barium compound and a zirconium compound together with ceria in bulk form. This is stated to enhance stability of the alumina washcoat upon exposure to high temperature.
In another approach, U.S. Pat. No. 4,233,189 teaches the use of non-alumina supports such as zirconia for rhodium, so that rhodium-alumina interaction can be avoided. However, zirconia has a lower surface area than gamma alumina and itself is not a thermally stable support. Zirconia undergoes a phase transition between its monoclinic crystalline structure and its more stable tetragonal crystalline structure over a wide temperature range. Such transition causes drastic sintering of the associated precious metals. Thus, a need still exists for improved stabilization against thermal degradation of precious metals containing TWC catalysts.